The use of concrete railway ties in replacement of the standard wood ties is increasingly accepted in the construction of railways. However, the manufacture of individual prestressed concrete ties is expensive and time-consuming so that there is a need for providing a method and apparatus whereby a series of ties may be simultaneously formed for each concrete pouring operation, a method and apparatus which would also permit a high degree of mechanisation of the production processes.
An important requisite for the performance in service of a precast tie is the bond developed between the high tensile steel and the concrete; the bond must transfer the full prestressing force into the concrete as close as possible from the end of the tie to permit the presence of this force in the rail seat area. Pouring of the precast ties as a continuous piece of concrete provides the best conditions to develop the best possible bond. Furthermore because of the sawing operations, which cut the concrete as well as the wires, the end of the prestressed steel penetration from the concrete face can be measured and the anchor length of the prestressing force be computed. Each tie can also be visually inspected for the meeting of this essential requirement.
However, it is difficult to make the ties as a continuous piece of concrete due to the particular shape of a railway tie. Indeed, the monobloc concrete railway tie must include, as an integral part thereof, iron shoulders to anchor track fastening devices; therefore, the casting mould must be capable of holding these anchoring shoulders in position until the concrete is set. Secondly, the concrete railway tie must be prestressed; hence, the casting mould must be particularly constructed to allow it to move as the tension on the prestressing wires is transferred to the concrete.